Resistance spot welding, in general, relies on the resistance to the flow of an electrical current through contacting metal workpieces and across their faying interface to generate heat. To carry out such a welding process, a pair of opposed spot welding electrodes are typically clamped at diametrically aligned spots on opposite sides of the workpieces at a predetermined weld site. A momentary electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to this flow of this electrical current generates heat within the metal workpieces and at their faying interface (i.e., the contacting interface of the metal workpieces). The generated heat initiates a molten weld pool which, upon stoppage of the current flow, solidifies into a weld nugget. After the spot weld is formed, the welding electrodes are retracted from their respective workpiece surfaces, and the spot welding process is repeated at another weld site.
Resistance spot welding has long been used by a number of industries to join together two or more steel workpieces. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated bare or galvanized steel sheet layers during the manufacture of a vehicle body panel for a door, hood, trunk lid, or lift gate, among others. A number of spot welds are typically formed along a peripheral edge of the steel sheet layers or some other bonding region to ensure the body panel is structurally sound. Because of the recent push to incorporate lighter-weight materials into a vehicle body structure, there is interest in using at least one thin-gauge steel workpiece to fabricate vehicle body panels like the ones listed above.
Conventional bare or galvanized steel spot welding practices have typically employed a weld schedule in which a welding current of constant amperage is continuously passed through the steel workpieces to form the molten weld pool. In particular, a constant welding current lying somewhere between about 4 kA and 20 kA would usually be passed through the steel workpieces for a period of about 70 ms to about 700 ms. Such weld schedule parameters can consistently produce quality weld results so long as none of the steel workpieces being spot welded are less than about 0.8 mm in thickness. But if one or more of the steel workpieces has a thickness below 0.8 mm, down to about 0.6 mm, the prospects of consistently forming an acceptable spot weld begin to diminish with conventional weld schedules, and often necessitate that other enabling welding procedures be implemented such as, among others, as electrode dressing and stiffened gun arms.
Because of high part reject rates, spot welding steel workpieces in which one of the workpieces is less than 0.6 mm in thickness has long been considered unfeasible when using conventional spot welding equipment with a conventional weld schedule that specifies a constant amperage welding current. The main technical problem that proscribes this spot welding practice is the difficulty in controlling the initiation and growth of the molten weld pool in such a thin-gauge steel workpiece. Indeed, the rapid heat build-up in a steel workpiece less than 0.6 mm thick can produce a weld pool that rapidly achieves 100% penetration, thus leading to surface metal expulsion and/or electrode degradation. And in instances where a weld nugget is actually derived and does not burn through the thin-gauge steel workpiece—which is an unpredictable occurrence—the size, location, and structural integrity of the weld nugget is inconsistent. In light of these difficulties, steel workpieces that are less than 0.6 mm thick are usually joined to other workpieces by mechanical techniques such as clinching or self-piercing riveting.